This invention relates to an improved exhaust gas sensor of the type responsive to the partial pressure of oxygen in exhaust gases from an internal combustion engine. More specifically, the invention pertains to an exhaust gas sensor that uses titania or other ceramic elements to generate an electrical signal. The ceramic elements each have precious metal electrodes embedded therein for providing conductive electrical connection to the ceramic element. Resistance of the ceramic element varies as a function of the partial pressure of oxygen and/or temperature to which the ceramic element is exposed.
Exhaust gas oxygen sensors of this type are used in feedback fuel control systems for internal combustion engines. These fuel control systems utilize the step-function response of the sensor that results from similar step-function changes in the partial pressure of oxygen in the exhaust gases of an internal combustion engine as the air/fuel mixture supplied to the engine cycles between rich and lean with respect to the stoichiometric air/fuel ratio. In the titania exhaust gas sensor, the titania ceramic element has a low resistance when located in an environment containing a substantial amount of oxygen.
The preferred titania sensor consists not only of an element for measuring the partial pressure of oxygen, but also of a second element connected in series with the oxygen sensing element and used for purposes of providing temperature compensation of the sensor output voltage. In the preferred form, the oxygen sensing element is made from porous titania ceramic material and the temperature sensing element is a thermistor that may be made from a more dense titania ceramic material or from some other material such as praseodymium ferrite. The two elements typically are disc-shaped and each contains two precious metal electrode wires spaced apart from one another for use in measuring the electrical resistance between the electrode wires. The wires have in the past been made from pure platinum.
One of the failure modes of the exhaust gas sensors described above is due to degradation of the platinum electrode wires. The exhaust gas sensor in use is positioned in the exhaust system of an internal combustion engine. Usually the sensor is installed in an aperture at a location in the exhaust manifold near the flange that would connect to an exhaust pipe. The response of the metal oxide ceramic elements in the exhaust gas sensor to temperature and to the partial pressure of oxygen in the exhaust gases occurs over a normal temperature operating range that extends from about 350.degree. C. to about 850.degree. C. and which, in some cases, may extend from a lower temperature of 300.degree. C. to a higher temperature of 900.degree. C. The platinum electrode wires of the sensor are exposed to the exhaust gases and also are at temperatures within the ranges just mentioned. It is this hostile environment that produces the degradation of the platinum electrode wires of the prior art exhaust gas oxygen sensor.
According to the present invention, the durability of the platinum electrodes of an exhaust gas sensor is substantially improved.